High-temperature conversion processes as steam-cracking provide a wide range of unsaturated hydrocarbons, normally in gaseous state such as ethylene, propylene, butadiene and butenes, and in liquid state such as C.sub.5, C.sub.6, C.sub.7 olefins forming part of pyrolysis gasoline.
These products can be separated by mere fractionation, depending on their distillation range, thus providing C.sub.2, C.sub.3, C.sub.4 cuts and gasoline.
However, each of these cuts is a mixture of paraffinic, monoolefinic, diolefinic and acetylenic hydrocarbons.
For example, a typical composition of C.sub.4 cut is as follows:
______________________________________ Butadiene 20-50% by weight Butane 10-20% " Butenes 20-50% " Butynes 0.05-0.2% " Vinylacetylene 1-3% " ______________________________________
It may happen that the C.sub.4 cut contains a minor proportion of C.sub.3 or C.sub.5 hydrocarbons, for example up to 5% by weight.
Upgrading of butadiene contained in said cut is only achievable after removal of butyne and vinylacetylene.
This purification may be performed by: extractive distillation or/and selective hydrogenation.
Extractive distillation provides butadiene containing less than 100 ppm by weight of vinylacetylene but this limit is only achieved at the cost of a significant butadiene loss. As a matter of fact, fractionation of said cut purifies butadiene by increasing the concentration of acetylenics in a secondary effluent which, generally, is burnt.
The explosiveness of high concentrations of components in said secondary effluent requires dilution with products which could otherwise be upgraded such as butadiene and butenes, thus decreasing substantially the yield of the plant.
For this reason, a selective hydrogenation unit is generally provided, upstream of the extraction unit, which decreases the acetylenic content of the mixture to be separated, thus proportionally decreasing the butadiene losses due to dilution.
However, this improvement in butadiene yield is only substantial when hydrogenation is really selective.
This hydrogenation is generally performed in liquid phase at temperature of 10.degree.-80.degree. C., under pressures of 4-10 bars, over a palladium-containing catalyst.
The known processes now used result in such ion butadiene yields that the overall saving achieved by this additional hydrogenation step is marginal.